The present invention relates generally to integrated circuit heat dissipation devices, and, more particularly, to an anisotropic heat spreading apparatus and method for semiconductor devices.
The semiconductor industry has seen tremendous technological advances in recent years that have permitted dramatic increases in circuit density and complexity, as well as equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-chip microprocessors with many millions of transistors, operating at speeds of tens (or even hundreds) of MIPS (millions of instructions per second), to be packaged in relatively small, air-cooled semiconductor device packages. Because integrated circuit devices, microprocessors and other related components are designed with increased capabilities and increased speed, additional heat is generated from these components.
As packaged units and integrated circuit die sizes shrink, the amount of heat energy given off by a component for a given unit of surface area is also on the rise. The majority of the heat generated by a component, such as a microprocessor for example, must be removed from the component in order to keep the component at an acceptable or target operating temperature. If the heat generated is not removed from the component, the heat produced can drive the temperature of the component to levels that result in early failure of the component. In some instances, the full capability of certain components cannot be realized since the heat the component generates at the full capability would result in failure of the component.
An integrated circuit has a front side and a backside. The front side of the integrated circuit includes leads for inputs, outputs and power to the integrated circuit. Leads include many forms, including pins and balls in a ball grid array. The leads of an integrated circuit are attached to pads on another device such as a printed circuit board. For example, an integrated circuit that includes a die having a microprocessor therein has a front side that is attached to the pads on a motherboard, substrate or leadframe. In contrast, a heat sink is attached to the backside of the integrated circuit, extending away from the printed circuit board to which the integrated circuit is mounted. Accordingly, a major portion of the heat generated is generally extracted from the backside of the integrated circuit with the die therein.
There is a practical limitation on the amount of heat that can be extracted from the backside of the integrated circuit die, as a result of the thermal resistance of the thermal interface materials (such as any thermal grease, adhesives or solders) used between the backside of the integrated circuit die and the heat sink. Typically, heat sinks are formed from materials such as copper or aluminum and have a limited ability to conduct heat. Relatively large fin structures are also provided to increase the amount of heat removed via conduction. Fans are also provided to move air over the fin structures to aid in the removal of heat. Increasing the size of the fin structure increases the volume of the heat sink, and generally also increases the stack height of the heat sink. In many electronic devices, the overall size of the heat sink is generally limited by volume constraints of the housing. For example, in some mobile products such as laptop computers and ultra-mobile computers, small stack heights are required.
The use of aluminum and copper heat sinks with fin structures are now therefore approaching their practical limits for removal of heat from a high performance integrated circuit, such as the integrated circuits that include dies for microprocessors. When heat is not effectively dissipated, the dies develop “hot spots” (i.e., areas of localized overheating). Unfortunately, the current cost performance lids do not adequately solve this heat dissipation/distribution problem. Moreover, the existing lid materials are isotropic, in that they provide singular heat flow characteristics derived from the intrinsic homogeneous properties of the lid material. In some instances, traditional aluminum and copper heat sinks have been replaced with sinks incorporating exotic materials (e.g., diamond particles) therein. However, diamond heat sinks are difficult to manufacture, in addition to being expensive. In particular, one aspect of diamond heat sink formation is that one major surface of the heat sink must be ground smooth in order to provide a good thermal connection at a thermal interface. The grinding or smoothing of diamond is also time consuming.
In view of the above, it would be desirable to be able to provide a heat spreading apparatus and methodology for semiconductor devices in a manner that is both efficient and cost effective.